1. Field
Example embodiments relate to a method of manufacturing a ZnO-based thin film transistor (TFT), and more particularly, to a method of manufacturing a ZnO-based TFT which may effectively prevent or reduce damage to a channel layer.
2. Description of the Related Art
Televisions (TVs) may be the main applications of rapidly progressing flat panel displays. While liquid crystal displays (LCDs) are the most widely used displays for TVs, many attempts have been made to use organic light-emitting diodes (OLEDs) for TVs. The displays for TVs that have been developed are large screen, digital information display (DID), with relatively low cost, and high quality in moving image, resolution, brightness, contrast, and color production, which may be important considerations in the market. A substrate, e.g., a glass substrate, may increase in size and a thin film transistor (TFT) having improved characteristics may be used as a display switching and driving element without increasing costs. Given this trend, efforts may be made to develop TFTs for higher quality displays at a lower cost.
Amorphous-silicon (a-Si) TFTs, which may be uniformly formed on a relatively large substrate up to 2 m or larger at a lower cost, may now be widely used as driving and switching elements for displays. However, because displays become larger and have higher image quality and thus driving and switching devices may also be required to have increased performance, existing a-Si TFTs having a low electron mobility of approximately 0.5 cm2/Vs may suffer from limitations. Accordingly, high performance TFTs having an electron mobility higher than that of the a-Si TFTs may be developed. The a-Si TFTs have another problem in that as they may be operated for a long time, the performance of the a-Si TFTs may be degraded, thereby lowering reliability. For this reason, the a-Si TFTs may be more difficult to be applied to organic light emitting diodes (OLEDs), which emit light using continuously applied current, than to liquid crystal displays (LCDs).
Because poly-Si TFTs having much higher performance than that of a-Si TFTs have an increased mobility of tens to hundreds of cm2/Vs, the poly-Si TFTs may be applied to higher quality displays for which existing a-Si TFTs may be unsuitable. In addition, the degradation of poly-Si TFTs may be much less than that of a-Si TFTs. However, in order to manufacture poly-Si TFTs, more processes than those required to manufacture a-Si TFTs may be required and additional equipment may be required. Accordingly, because poly-Si TFTs may be economically inferior to a-Si TFTs although the poly-Si TFTs may be suitable for higher quality displays or OLEDs, the poly-Si TFTs may suffer from limitations as well. Because poly-Si TFTs have not been manufactured on a relatively large substrate exceeding 1 m because of technical problems, e.g., equipment needs or undesirable uniformity, the poly-Si TFTs may be difficult to be applied to TVs, thereby making it more difficult for high performance poly-Si TFTs to survive and prosper in the market.
Accordingly, the demand for TFTs having a relatively large design, lower cost, and increased uniformity, which may be the advantages of a-Si TFTs, and increased performance and reliability, which may be the advantages of a poly-Si TFTS, may increase and research on the TFTs is being actively performed, e.g., research on an oxide semiconductor. Recently, ZnO-based TFTs have received increasing attention as oxide semiconductor devices. ZnO-based TFTs may include a zinc oxide (ZnOx) TFT, and a gallium-indium-zinc-oxide (GIZO) TFT that may be a mixture of gallium oxide (GaOx), indium oxide (InOx), and zinc oxide (ZnOx). According to the related art, while a ZnOx TFT, which may be polycrystalline, has an increased electron mobility similar to that of a poly-Si TFT but has lower uniformity, a GIZO TFT, which is amorphous, may have improved characteristics compared to those of a a-Si TFT and may offer both the advantages of the a-Si TFT and the poly-Si TFT because the GIZO TFT may be manufactured in the same manner as the a-Si TFT. However, any definite method of manufacturing a GIZO TFT has not been suggested and there may be several technical problems in manufacturing. A bottom gate back channel etching (BCE) TFT may be more desirable for several known reasons.
Because a GIZO semiconductor film may be amorphous, the GIZO semiconductor film may be processed at a lower temperature and may be more easily made larger. However, carrier concentration in a ZnO-based semiconductor film including the GIZO semiconductor film may be sensitive to a change in oxygen concentration, and the physical and electrical properties of the ZnO-based semiconductor film may be affected by thermal and chemical shock. When a ZnO-based TFT is manufactured, the ZnO-based semiconductor film may be exposed to high-energy plasma. The semiconductor film may suffer a defect, e.g., an oxygen vacancy, due to the decomposition of ZnO, resulting in an increase in the carrier concentration.
For example, when a bottom gate GIZO-based transistor is manufactured, a process of forming source and drain electrodes may be important, and the selectivity of a channel layer to a gate insulating layer may be particularly important in the process of forming the source and drain electrodes. If the channel layer under the source and drain electrodes is lost or the gate insulating layer outside the source and drain electrodes is over-etched and lost in the process of forming the source and drain electrodes, the bottom gate GIZO-based TFT may suffer undesirable performance and non-uniformity.
The source and drain electrodes may be typically formed by plasma etching. During plasma etching, a surface of the channel layer, e.g., a back channel, may be damaged, bonding between oxygen and each of Ga, In, and Zn may be broken down, and electrical conductivity may be increased. The increase in the electrical conductivity may make it difficult to control the threshold voltage of the bottom gate GIZO-based TFT and may decrease an on-off current ratio Ion/Ioff. To solve the problems, attempts have been made to restore the bonding between the oxygen and each of the Ga, In, and Zn through post-annealing or additional oxygen supply. However, such attempts have failed to correct the problems. Also, when a passivation layer, which may be a protective layer of the GIZO-based TFT, is deposited after the source and drain electrodes are formed, the surface of the channel layer may be damaged by plasma again.